Use of optical storage devices has become common since the advent of the compact disc (CD) widely used for the storage of music, video and other information. Optical storage devices of this type require a transparent substrate with excellent optical properties. This substrate is encoded with information often by molding in a series of pits or depressions. Suitably coated this substrate can be read by a laser to give a series of signals recovering the information stored on the disc. With storage devices of this type, there is a growing need to store more and more information in a smaller space.
Bisphenol A Polycarbonate (BPA-PC) has been widely used for optical storage media applications, however, BPA-PC has some limitations. It is rather difficult to process by injection molding which limits the speed with which discs can be made and the quality and amount of information that can be stored on them. In these applications BPA-PC and optical data storage devices made from it are limited by their birefringence. Birefringence, resulting from the inherent properties of the resin and also from how it was processed (influenced by its rheological properties) can interfere with the recovery of information stored on the device (i.e. disc).
In the further development of optical discs, particularly read-write discs and discs which are capable of storing larger amounts of data, various physical factors become increasingly important. One such factor, which is closely related to the storage capacity of the disc, is birefringence, i.e., the difference between indices of refraction for light polarized in perpendicular directions. Birefringence leads to phase retardation between different polarization components of the laser beam (i.e., a polarization-dependent effect), thereby reducing readability of the disc.
Birefringence has several sources including the chemical nature of the raw material from which the disc is fabricated, the degree of molecular orientation therein, and thermal stresses in a fabricated plastic optical disc. The observed birefringence of a disc is therefore determined by the molecular structure, which determines the intrinsic birefringence, and the processing conditions, which can create thermal stresses and orientation of the polymer chains. Specifically, the observed birefringence is typically a function of the intrinsic birefringence plus the birefringence introduced upon molding articles such as optical discs.
Structural variations of BPA-PC have been made to deal with the birefringence limitations of BPA-PC but many of them do not fully meet the other requirements for a successful optical data storage device material. They are either too brittle, have poor optical properties (low transmittance and /or high haze), or are difficult to process due to their high glass transition temperature (Tg). High processing temperature can also lead to degradation of the polymer chain leading to loss of mechanical properties, color formation (especially yellowing) and generation of gaseous by-products impairing optical properties. Other potential optical materials of low birefringence do not meet the needs of an optical storage device because they are too floppy (have a flex modulus below about 150,000 psi) or have a low thermal capability (Tg below about 80.degree. C.).
Therefore, there is a need to prepare resin compositions and articles made from them that are transparent, have low birefringence and good melt processability.
There are several patents describing specific types of aromatic polycarbonate with improved optical properties or higher thermal capability. Polycarbonates of a specific molecular weight range with at least one pendant aromatic group and an optical disc substrate made thereof are claimed by M. Hasuo et al. in U.S. Pat. No. 4,734,488. These materials are shown to have superior heat resistance (higher Tg) than polycarbonate along with good optical properties.
U.S. Pat. No. 4,680,374 claims an optical substrate with double refraction not greater than 5.times.10-5 made of a polycarbonate copolymer of aliphatic substituted bisphenols.
Polycarbonate polymers and copolymers of spiro dihydric phenols and their preparation are disclosed by V. Mark in U.S. Pat. No. 4,552,949 as exhibiting improved heat distortion and retaining transparency. The chain stiffness of these types of polycarbonates is discussed by R. Wimberger-Friedl, M. G. T. Hut and H. F. M. Schoo in Macromolecules, 29, 5453-5458 (1996).
Specific spiro biindane aliphatic diacid copolymers are disclosed as having low birefringence in published EP 846711-A2 entitled Optical Disk grade Copolyestercarbonates Derived from Hydroxyphenyl Indanols.
There are references to transparent blends of aromatic polycarbonates with specific cycloaliphatic polyesters but none address birefringence or the requirements of optical storage devices.
U.S. Pat. No. 4,188,314 describes shaped articles (such as sheet and helmets) of blends of 25-98 parts by weight (pbw) of an aromatic polycarbonate and 2-75 pbw of a poly cyclohexane dimethanol phthalate where the phthalate is from 5-95% isophthalate and 95-10% terephthalate. Articles with enhanced solvent resistance and comparable optical properties and impact to the base polycarbonate resin and superior optical properties to an article shaped from a polycarbonate and an aromatic polyester, such as polyalkylene terephthalate, are disclosed.
There are other patents that deal with polycarbonate polycyclohexane dimethanol phthalate blends for example; U.S. Pat. No. 4,125,572; U.S. Pat. Nos. 4,391,954; 4,786,692; 4,897,453 and 5,478,896. U.S. Pat. No. 5,478,896 relates to transparent polycarbonate blends with 10-99% polyester of CHDM with some minor amount of aliphatic diol and iso and terephthalic acid. U.S. Pat. No. 4,786,692 relates to a 2-98% aromatic polycarbonate blend with a polyester made of cyclohexane dimethanol (CHDM) and ethylene glycol (EG) in a 1:1 to 4:1 ratio with iso and terephthalic acid. U.S. Pat. No. 4,391,954 describes compatible compositions of non halogen polycarbonate (PC) and amorphous polyesters of CHDM and a specific iso/tere phthalate mixture. U.S. Pat. No. 4,125,572 relates to a blend of 40-95% PC, 5-60% polybutylene terephthalate (PBT) 1-60% and 1-60% an aliphatic/cycloaliphatic iso/terephthalate resin. US 4,897,453 describes blends of 10-90 % PC, 10-90% of a polyester of 0.8-1.5 IV, comprised of 1,4-cyclohexane dicarboxylic acid, 70% trans isomer, CHDM and 15-50 wt. % poly oxytetramethylene glycol with 0-1.5 mole % branching agent. Also claimed are molded or extruded articles of the composition. None of these references raise, suggest, or address the question of birefringence and the special needs for an optical data storage material.